We also want to highlight the importance of the principal tensor as reviewed in [1]}. Spacetimes
possessing a principal tensor are always algebraically special (type D). If they are vacuum solutions, then they
also have aligned fields: electromagnetic fields with the same components as the principal tensor. These fields have \((D-\epsilon )/2\)  free parameters; setting these charges proportional to the mass and \((D-2-\epsilon )/2\)  NUT charges results in our single copy field. In some sense the principal tensor controls the form of the metric, so it is not too surprising that it is related so closely to the metric's single copy. A natural extension of our work would be to include non-vacuum metrics which still possess a principal tensor.
